This invention relates to atomic frequency standard cells such as cesium and rubidium cells, and to methods for fabricating such cells.
Optically pumped atomic frequency standards using alkali metal such as cesium and rubidium vapor have come into wide spread use as basic frequency standards. Weidemann U.S. Pat. No. 4,661,782 provides a general introduction to such frequency standards. Generally speaking, a gas cell is provided within a resonant microwave cavity. In a rubidium gas cell frequency standard, for example, microwave resonance in the rubidium is detected by measuring the absorption of a suitable optical radiation which is directed at the gas cell while the resonant cavity is excited by externally generated microwave radiation. The frequency of this microwave radiation is controlled according to the degree of absorption of the optical radiation.
When permanent gases such as helium, hydrogen, and nitrogen are present inside a reference cell containing the rubidium atoms, these gases act to shift (increase or decrease) the atomic reference frequency. (A "permanent gas" is a substance that exists completely in the vapor phase at all temperatures and pressures of interest.) The amount of the shift depends on the type of gas and the density of the gas in the cell and is therefore proportional to the partial pressure of the gas in the cell.
At 100.degree. C. and below, glass is essentially impermeable to most gases. Helium and hydrogen are two exceptions to this statement. Since the helium atom has greater permeability than the hydrogen molecule, glass is much more permeable to helium than to hydrogen, and, helium that is normally present in the atmosphere in small amounts can slowly permeate a glass cell. (Hydrogen is also present in the atmosphere, but at ten times lower concentration than helium). This results in a helium concentration inside the cell that changes with time.
If the cell initially has no helium inside, then helium from the atmosphere will slowly pass through the glass and the helium density inside the cell will increase with time. The density increases to a final steady-state value which is equal to the density of helium in the atmosphere. At this point the permeation of helium through the glass stops because there is no longer any pressure differential across the glass walls of the cell. Typically this process takes years to stabilize, and the permeation rate is largest at the start.
The accumulation of helium inside the cell produces a frequency shift in the output frequency of the standard that varies with time. This behavior contributes to the overall frequency aging of the standard. Such aging is highly undesirable, because one of the reasons for using a rubidium frequency standard is that it exhibits low aging compared to less expensive devices, such as stand-alone crystal oscillators.
The rate at which helium accumulates in a cell (atoms/cc/sec) depends on the total permeation rate (atoms/sec) and the volume. The permeation rate is proportional to the total surface area of the cell. Generally speaking, as a cell is made smaller, its volume to surface area ratio decreases; i.e., the volume fills up faster with helium in a smaller cell because there is proportionally more surface area available for permeation. Thus, as cells are made smaller, the need for glass with lower helium permeability becomes more important.
Various approaches have been proposed for containing alkali metal vapor in a suitable cell, as discussed in the following U.S. Patents:
______________________________________ U.S. Pat. No. Inventor ______________________________________ 3,242,423 L. Malnar 3,248,666 D. J. Farmer 3,510,758 G. R. Huggett 3,577,069 L. Malnar 3,675,067 H. Brun 4,405,905 Busca et al. 4,494,085 S. Goldberg 4,569,962 H. Robinson ______________________________________
The cells described in the Malnar, Brun, Busca, and Huggett patents all rely on cells that are blown from a suitable glass. This approach brings with it a number of important disadvantages. First, skilled glass-blowers are needed to form such cells, and it is progressively more difficult to achieve uniform dimensions of the cells as they are made smaller. Furthermore, when gas cells are made smaller there is an increased importance in using low helium permeability glasses, and some low helium permeability glasses are difficult to work using glass-blowing techniques.
Another approach discussed in the Farmer patent is to eliminate the glass cell entirely and instead to supply windows in the metal walls of the microwave resonant cavity. The Farmer patent expressly recognizes the difficulty in manufacturing gas cells of glass or quartz to sufficiently close tolerances, and attempts to avoid these problems by eliminating the glass cell entirely. In the resonating cavity of Farmer, the windows 16, 17 are of glass, and they may be sealed to the resonator 10 by glass to metal seals using fusible rings attached to the cylindrical extensions 18, 20. The approach taken in the Farmer patent allows the alkali vapor gas to come into contact with the metal walls of the resonating cavity. Furthermore, it is not possible to replace the gas without simultaneously replacing the resonating cavity.